Microchannel plates (MCPs) are thin wafers of electrically conducting glass which contain thousands of open channels or tubes. Each channel diameter is on the order of 10 to 15 microns. The plates are used for the amplification of X-rays, ions, or electrons. They are an essential component of a variety of electronic devices, including night vision goggles.
The principles of microchannel plate fabrication and operation are described in detail in the literature. For example, see an article by Michael Lampton in Scientific American, 245, 62-71 (1981). A fabrication procedure in current use is described in detail in U.S. Pat. Nos. 4,629,486 (Uchiyama et al.) and 4,112,170 (Rauscher). The former is particularly concerned with alkali lead silicate cladding glasses; the latter with barium borosilicate core glasses.
Briefly, the method, as there described, involves fusing solid rods of a leachable core material, preferably glass, within tubular pieces of relatively non-leachable skin glass. A bundle of such composite bodies is formed and fusion sealed. The bundle is then drawn down, cut into lengths, rebundled and further drawn. Ultimately, a composite article is obtained in which an interconnected glass matrix of the skin glass encases an array of leachable core elements. The composite is then exposed to a leachant, for example, hydrochloric or nitric acid, to remove the cores. This leaves the skin glass matrix with an array of channels corresponding to the array of core elements. The channels may be on the order of ten microns in diameter. The perforated plate, thus formed, is then heated in a hydrogen-containing atmosphere to produce a surface layer of reduced metal on the channel walls. As used throughout, the term "perforated plate," or face plate, means a plate containing an array of channels, as just described.
A cladding glass, suitable for MCP production, must possess a particular combination of physical and electrical properties. Initially, the glass must have a liquidus viscosity suitable for drawing tubing. A viscosity in excess of 30,000 poises at the internal liquidus is acceptable. However, a viscosity over 100,000 poises is preferred.
Another requirement is compatibility with soluble core glasses. During microchannel plate fabrication, a soluble core glass is inserted inside the cladding glass tubing to maintain dimensional stability through redraw. The core glass is ultimately leached from the channels. As a consequence, any cladding glass requires lower softening point and thermal coefficient of expansion (CTE) than the soluble core glass, and about 10.sup.4 lower etch rate. Fortunately, a wide range of soluble core glasses is available, for example, those disclosed in the Rauscher patent mentioned earlier.
The glass must also have a suitable surface resistivity, a value of about 10.sup.13 ohms/sq. normally being desired. However, some variation can usually be tolerated, and surface resistivities between 10.sup.11 and 10.sup.14 ohms/square are considered acceptable. Typically, a lead silicate glass is employed, and fired in hydrogen at 400-500.degree. C. to obtain an appropriate surface resistivity.
A key consideration, of course, is a high secondary electron emission constant. As a practical matter, the stability of the secondary emission is of even greater significance. That determines the useful life of an MCP device.
In the past, alkali lead silicate glasses have been widely used in MCPs. Stability of secondary electron emission was a problem, however, leading to shortened life. Studies at Mullard Research Laboratories in England have shown that ion migration may affect stability. Accordingly, efforts have been made to substitute larger alkali ions, such as cesium or rubidium, for smaller ions, such as potassium and sodium, in the lead silicate glass composition. While this appears to have helped, the problem still persists.
Outgassing of both H.sub.2 and CO.sub.2 during MCP usage is a primary cause of reduced signal/noise. This is primarily because the products of outgassing are accelerated inside individual MCP channels due to the electric field. This results in spurious electronic emission.
To eliminate outgassing, MCPs are generally heated at relatively high temperatures prior to installation in a device. A possible solution, then, is to provide a harder MCP glass, that is, a glass that softens at higher temperatures. Such a glass would permit higher heat treating temperatures. This should increase gas diffusion from the glass, and, consequently, reduce outgassing in use.